Solutions to replace Qt's signal-slot connections (across thread boundaries)? - c++

In Qt one can:
connect(object, &Object::someSignal, objectInAnotherThread, &Object::someSlot);
So, when I connect a signal from an object in a thread to an object in another thread, Qt queues the signal and someSlot will be executed in the thread of objectInAnotherThread.
This particular feature is very handy and safe, although could copy data.
Lambdas in C++11 are handy, but when replacing this kind of connection with a pure lambda callback (without Qt), the lambda will be executed in the thread of the caller. This will then usually require mutexes etc error-prone logic to make things right.
I'm aware of Boost::signals2 etc, but AFAIK they don't provide this same Qt-like behavior when used across thread boundaries..?
If I'd like to remove Qt for a reason or another, what are my options for drop-in replacement regarding my signal-slot connections?

What’s wrong with spinning up a thread and sending wrapped function calls to a queue that the thread pulls from and executes? The event queue in Qt is not very special other than it uses the “native” event loop. There’s no need to do that, though, and e.g. QtConcurrent::run threads implement a simple mutex+wait condition protected queue. Whenever the new events are delivered, the thread gets woken up and processes them until the queue is empty. The events can carry functor calls. In fact, the events can simply be std::function. The only sticking point is timers, which you’d have to implement on top of the primitive that waits on the wait condition. Those waits have timeouts, and you’d use a sorted timeout queue and schedule wake ups whenever a timer object should “tick”. This has the benefit of not using up any native timers and can potentially perform better.

Related

Should I use signal/slot as much as I can in Qt?

From Qt documentation on Performance Considerations And Suggestions I got the following:
use asynchronous, event-driven programming wherever possible
I'm not sure what that means, so would like to ask. Does it mean I should use signal/slots whenever possible (because they are asynchronous?)?
Qt signals/slots are not necessarily asynchronous. From https://doc.qt.io/qt-5/threads-qobject.html:
Direct Connection: The slot is invoked immediately, when the signal is emitted. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread.
Queued Connection: The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.
Blocking Queued Connection: The slot is invoked as for the Queued Connection, except the current thread blocks until the slot returns.
A signal subscribed to by a slot with a Direct Connection will be essentially a method call that you can "hook up" at runtime.
Also, yes, you should probably use "asynchronous, event-driven programming" "whenever possible" for a sane definition of "whenever possible".
Obviously, don't replace all method calls between your objects with signals and slots. And when you do use signals and slots, don't always make them asynchronous (Queued) - sometimes you will want the objects subscribed to your signals to finish their "reactions" to your signals before the emitting function proceeds.
In general, when you don't really care if the subscribers of your signals get their slots invoked immedtiatelly or later, just connect them up without specifying a connection type, and Qt will use Auto Connection, which will do the right thing (thread-wise). When you do care, just specify the type of connection you want.
If you feel confused by this at first, a reasonable thing to do might also be to make all connections Queued by default - you won't really notice any performance difference, and this might prevent you from accidentally writing code that depends on the slots executing "directly", when that was not your intent.
The suggestion in your link is mainly meant for any events that get generated on your main thread, most likely by UI elements - buttons, etc. The main idea is that you want to process any input events as quickly as possible, to keep the main thread free for accepting any later events and rendering your UI, and, if the events cause any significant work to be done, move that work to another thread, and have your main thread wait for a completion signal, so that your main thread remains "responsive". If you want your UI to immedtially react to any events, for example, by initiating a "loading spinner" or displaying a progress bar, you can, of course, do that directly. This, of course, also applies to any other threads that might need to remain responsive and handle other events while a larger calculation is happening in the background.

Notify caller that a thread has finished

I am trying to use the multithreading features in the C++11 standard library and have the following situation envisioned.
I have a parent class which maintains a queue of thread. So something like:
std::queue<MyMTObject *> _my_threads;
The class MyMTObject contains the std::thread object.
The queue has a fixed size of 5 and the class initially starts with the queue being full.
As I have jobs to process I launch threads and I remove them from the queue. What I would like is to get a notification when the job is finished along with the pointer to the MyMTObject, so that I can reinsert them into the queue and make them available again.
I have basically 2 questions:
1: Is this a sound idea? I know I have not specified specifics but broadly speaking. I will, of course, control all access to the queue with a mutex.
2: Is there a way to implement this notification mechanism without using external libraries like Qt or boost.
For duplicates, I did look on the site but could not find anything that was suitable to manage a collection of threads.
I'm not sure if I need to mention this, but std::thread objects can't be re-used. Generally, the only reason you keep a std::thread reference is to std::thread::join the thread. If you don't plan to join the thread later (e.g. dispatch to threads and wait for completion), it's generally advised to std::thread::detach it.
If you're trying to keep threads for a thread pool, it's probably easier to have each thread block on the std::queue and pull objects from the queue to work on. This is relatively easy to implement using a std::mutex and a std::condition_variable. It generally gives good throughput, but to get finer control over scheduling you can do things like keep a seperate std::queue for each thread.
Detaching the threads and creating a work queue also has the added benefit that it avoids redundantly requesting the operating system create new threads which adds overhead and increases overall resource usage.
You could try to deploy some version of Reactor pattern I think. So, you could start one additional control thread that cleans after these workers. Now, you create a ThreadSafeQueue that will be used to communicate events from worker threads to control thread. This queue should be implemented in such a way that you can select on it and wait for any activity on the other end (some thread terminates and calls queue.push for example).
All in all I think it's quite elegant solution. I does add an overhead of an additional thread, but this thread will be mostly sleeping and waking up only once a while to clean up after the worker.
There is no elegant way to do this in Posix, and C++ threading model is almost a thin wrapper on Posix.
You can join a specific thread (one at a time), or you can wait on futures - again, one future at a time.
The best you can do to avoid looping is to employ a conditional variable, and make all threads singal on it (as well as indicating which one just exited by setting some sort of per-thread flag) just before they are about to exit. The 'reaper' would notice the signal and check the flags.
The issue is that this solution requires thread cooperation. But I know not of any better.

How to implement a function like invokeOnMainThread() in C/C++ linux?

I'm looking for a way to implement a functional equivalent of invokeOnMainThread(c# xamarin)/ runOnUiThread(android)/ performSelectorOnMainThread:(objective C) in C/C++ on linux.
Essentially what I need is a function, which can be called from any thread, to which I pass a handler, which is executed on the main thread.
I believe this will be helpful in cases where I want to restrict access to a particular resource to a particular thread (eg. libmysql DB access to main thread).
What would be an elegant way to implement this?
How is it done in android?
It is straightforward to have your runOnMainThread function place handlers in a thread-safe queue for later execution by the main thread. Your main thread needs to be prepared to periodically execute handlers it finds in that queue. This implies that the main thread needs to include a loop that periodically checks for new handlers in the queue. This periodic checking can be made efficient through the use of semaphores or other thread-safe signaling mechanisms.
Here are some SO questions on thread safe queues:
C++11 thread-safe queue
is std::queue thread safe with producer and multiple consumers

Qt signals (QueuedConnection and DirectConnection)

I'm having trouble with Qt signals.
I don't understand how DirectConnection and QueuedConnection works?
I'd be thankful if someone will explain when to use which of these (sample code would be appreciated).
You won't see much of a difference unless you're working with objects having different thread affinities. Let's say you have QObjects A and B and they're both attached to different threads. A has a signal called somethingChanged() and B has a slot called handleChange().
If you use a direct connection
connect( A, SIGNAL(somethingChanged()), B, SLOT(handleChange()), Qt::DirectConnection );
the method handleChange() will actually run in the A's thread. Basically, it's as if emitting the signal calls the slot method "directly". If B::handleChange() isn't thread-safe, this can cause some (difficult to locate) bugs. At the very least, you're missing out on the benefits of the extra thread.
If you change the connection method to Qt::QueuedConnection (or, in this case, let Qt decide which method to use), things get more interesting. Assuming B's thread is running an event loop, emitting the signal will post an event to B's event loop. The event loop queues the event, and eventually invokes the slot method whenever control returns to it (it being the event loop). This makes it pretty easy to deal with communication between/among threads in Qt (again, assuming your threads are running their own local event loops). You don't have to worry about locks, etc. because the event loop serializes the slot invocations.
Note: If you don't know how to change a QObject's thread affinity, look into QObject::moveToThread. That should get you started.
Edit
I should clarify my opening sentence. It does make a difference if you specify a queued connection - even for two objects on the same thread. The event is still posted to the thread's event loop. So, the method call is still asynchronous, meaning it can be delayed in unpredictable ways (depending on any other events the loop may need to process). However, if you don't specify a connection method, the direct method is automatically used for connections between objects on the same thread (at least it is in Qt 4.8).
in addition to Jacob Robbins answer:
the statement "You won't see much of a difference unless you're working with objects having different thread affinities" is wrong;
emitting a signal to a direct connection within the same thread will execute the slot immediately, just like a simple function call.
emitting a signal to a queued connection within the same thread will enqueue the call into the threads event loop, thus the execution will always happen delayed.
QObject based class has a queued connection to itself
Jacob's answer is awesome. I'd just like to add a comparative example to Embedded Programming.
Coming from an embedded RTOS/ISR background, it was helpful to see the similarities in Qt's DirectConnection to Preemptive behavior of the ISRs and Qt's QueuedConnection to Queued Messages in an RTOS between tasks.
Side note: Coming from an Embedded background, it's difficult for me to not define the behavior in the programming. I never leave the argument as Auto, but that is just a personal opinion. I prefer everything to be explicitly written, and yes that gets difficult at times!

using boost sockets, do I need only one io_service?

having several connections in several different threads.. I'm basically doing a base class that uses boost/asio.hpp and the tcp stuff there..
now i was reading this: http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/tutorial/tutdaytime1.html
it says that "All programs that use asio need to have at least one io_service object."
so should my base class has a static io_service (which means there will be only 1 for all the program and a all the different threads and connections will use the same io_service object)
or make each connection its own io_service?
thanks in front!
update:
OK so basically what I wish to do is a class for a basic client which will have a socket n it.
For each socket I'm going to have a thread that always-receives and a different thread that sometimes sends packets.
after looking in here: www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/reference/ip__tcp/socket.html (cant make hyperlink since im new here.. so only 1 hyperling per post) I can see that socket class isn't entirely thread-safe..
so 2 questions:
1. Based on the design I just wrote, do I need 1 io_service for all the sockets (meaning make it a static class member) or I should have one for each?
2. How can I make it thread-safe to do? should I put it inside a "thread safe environment" meaning making a new socket class that has mutexes and stuff that doesn't let u send and receive at the same time or you have other suggestions?
3. Maybe I should go on a asynch design? (ofc each socket will have a different thread but the sending and receiving would be on the same thread?)
just to clarify: im doing a tcp client that connects to a lot of servers.
You need to decide first which style of socket communication you are going to use:
synchronous - means that all low-level operations are blocking, and typically you need a thread for the accept, and then threads (read thread or io_service) to handle each client.
asynchronous - means that all low-level operations are non-blocking, and here you only need a single thread (io_service), and you need to be able to handle callbacks when certain things happen (i.e. accepts, partial writes, result of reads etc.)
Advantage of approach 1 is that it's a lot simpler to code (??) than 2, however I find that 2 is most flexible, and in fact with 2, by default you have a single threaded application (internally the event callbacks are done in a separate thread to the main dispatching thread), downside of 2 of course is that your processing delay hits the next read/write operations... Of course you can make multi-threaded applications with approach 2, but not vice-versa (i.e. single threaded with 1) - hence the flexibility...
So, fundamentally, it all depends on the selection of style...
EDIT: updated for the new information, this is quite long, I can't be bothered to write the code, there is plenty in the boost docs, I'll simply describe what is happening for your benefit...
[main thread]
- declare an instance of io_service
- for each of the servers you are connecting to (I'm assuming that this information is available at start), create a class (say ServerConnection), and in this class, create a tcp::socket using the same io_service instance from above, and in the constructor itself, call async_connect, NOTE: this call is a scheduling a request for connect rather than the real connection operation (this doesn't happen till later)
- once all the ServerConnection objects (and their respective async_connects queued up), call run() on the instance of io_service. Now the main thread is blocked dispatching events in the io_service queue.
[asio thread] io_service by default has a thread in which scheduled events are invoked, you don't control this thread, and to implement a "multi-threaded" program, you can increase the number of threads that the io_service uses, but for the moment stick with one, it will make your life simple...
asio will invoke methods in your ServerConnection class depending on which events are ready from the scheduled list. The first event you queued up (before calling run()) was async_connect, now asio will call you back when a connection is established to a server, typically, you will implement a handle_connect method which will get called (you pass the method in to the async_connect call). On handle_connect, all you have to do is schedule the next request - in this case, you want to read some data (potentially from this socket), so you call async_read_some and pass in a function to be notified when there is data. Once done, then the main asio dispatch thread will continue dispatching other events which are ready (this could be the other connect requests or even the async_read_some requests that you added).
Let's say you get called because there is some data on one of the server sockets, this is passed to you via your handler for async_read_some - you can then process this data, do as you need to, but and this is the most important bit - once done, schedule the next async_read_some, this way asio will deliver more data as it becomes available. VERY IMPORTANT NOTE: if you no longer schedule any requests (i.e. exit from the handler without queueing), then the io_service will run out of events to dispatch, and run() (which you called in the main thread) will end.
Now, as for writing, this is slightly trickier. If all your writes are done as part of the handling of data from a read call (i.e. in the asio thread), then you don't need to worry about locking (unless your io_service has multiple threads), else in your write method, append the data to a buffer, and schedule an async_write_some request (with a write_handler that will get called when the buffer is written, either partially or completely). When asio handles this request, it will invoke your handler once the data is written and you have the option of calling async_write_some again if there is more data left in the buffer or if none, you don't have to bother scheduling a write. At this point, I will mention one technique, consider double buffering - I'll leave it at that. If you have a completely different thread that is outside of the io_service and you want to write, you must call the io_service::post method and pass in a method to execute (in your ServerConnection class) along with the data, the io_service will then invoke this method when it can, and within that method, you can then buffer the data and optionally call async_write_some if a write is currently not in progress.
Now there is one VERY important thing that you must be careful about, you must NEVER schedule async_read_some or async_write_some if there is already one in progress, i.e. let's say you called async_read_some on a socket, until this event is invoked by asio, you must not schedule another async_read_some, else you'll have lots of crap in your buffers!
A good starting point is the asio chat server/client that you find in the boost docs, it shows how the async_xxx methods are used. And keep this in mind, all async_xxx calls return immediately (within some tens of microseconds), so there are no blocking operations, it all happens asynchronously. http://www.boost.org/doc/libs/1_39_0/doc/html/boost_asio/example/chat/chat_client.cpp, is the example I was referring to.
Now if you find that performance of this mechanism is too slow and you want to have threading, all you need to do is increase the number of threads that are available to the main io_service and implement the appropriate locking in your read/write methods in ServerConnection and you're done.
For asynchronous operations, you should use a single io_service object for the entire program. Whether its a static member of a class, or instantiated elsewhere is up to you. Multiple threads can invoke its run method, this is described in Inverse's answer.
Multiple threads may call
io_service::run() to set up a pool of
threads from which completion handlers
may be invoked. This approach may also
be used with io_service::post() to use
a means to perform any computational
tasks across a thread pool.
Note that all threads that have joined
an io_service's pool are considered
equivalent, and the io_service may
distribute work across them in an
arbitrary fashion.
if you have handlers that are not thread safe, read about strands.
A strand is defined as a strictly
sequential invocation of event
handlers (i.e. no concurrent
invocation). Use of strands allows
execution of code in a multithreaded
program without the need for explicit
locking (e.g. using mutexes).
The io_service is what invokes all the handler functions for you connections. So you should have one running for thread in order to distribute the work across threads. Here is a page explain the io_service and threads:
Threads and Boost.Asio